In a conventionally known linear motion guide unit, rolling elements circulate through a circulation passage of a slider. In such a linear motion guide unit, in order for the rolling elements to smoothly enter or exit from a load-carrying race formed by a raceway groove of a carriage of the slider, gently sloped crownings are formed in regions of opposite ends of the raceway groove of the carriage, the opposite ends of the raceway groove being inlet portions; i.e., mouths, of the load-carrying race. The crownings are linear or curved inclined surfaces formed at opposite end portions of the raceway surface of the raceway groove of the carriage. In the linear motion guide unit, at a connection portion of the carriage to an end cap between the load-carrying race and a turnaround passage, a linear or curved chamfered portion is formed on an end surface of the carriage, the end surface extending from the crowning toward an inner circumferential portion of the turnaround passage formed in the end cap; i.e., the chamfered portion is provided at a distal end portion of the crowning.
According to a known guide apparatus, in order to achieve high-speed motion of a slider on a guide rail, a snag is removed from a course of the rolling elements which roll from a no-load area into a load area, thereby lowering sliding resistance and noise in the case of a high-speed relative motion between the slider and the guide rail. In the guide apparatus, a load-carrying rolling-element rolling groove has crowning areas at respective opposite ends of the load area; as the load-carrying rolling-element rolling groove and a rolling-element rolling groove approach a turnaround passage, the distance between the load-carrying rolling-element rolling groove and the rolling-element rolling groove gradually increases such that an edge portion of a load-carrying rolling-element rolling surface is recessed downward in relation to the inner side wall surface of the turnaround passage to thereby form a step portion; as a result, even in the presence of a dimensional error in forming a guide portion and in forming the load-carrying rolling-element rolling groove, the surface of the load-carrying rolling-element rolling groove does not protrude toward the guide rail in relation to the side wall surface of the turnaround passage; and the size of the step portion in relation to an end cap of a carriage is about 5% of the rolling-element diameter (see, for example, Japanese Patent Application Laid-Open No. 2002-155936).
A known linear motion guide unit includes a guide rail which has a rail-side rolling-element raceway groove formed therein along an axial direction, and a slider body which has a slider-side rolling-element raceway groove formed therein and facing the rail-side rolling-element raceway groove and can move in the axial direction via a plurality of rolling elements disposed in a rolling-element rolling passage composed of the rail-side rolling-element raceway groove and the slider-side rolling-element raceway groove. In the linear motion guide unit, the slider-side rolling-element raceway groove is partially formed of a first crowning, a second crowning sloped more steeply than the first crowning, and a sloped surface sloped more steeply than the second crowning, and an intersection point of an imaginary line extending linearly along the second crowning and an imaginary linear line extending along an end surface of the slider body coincides with an intersection point of an imaginary line extending arcuately along the outer circumference of an inner circumferential guide member which serves as the inner circumferential surface of a turnaround passage, and an imaginary line extending linearly along the end surface of the slider body (see, for example, Japanese Patent Application Laid-Open No. 2008-133837).
A linear motion guide unit which exhibits low noise and operates well has been known. In the linear motion guide unit, a groove of a return guide and a raceway groove of a slider are not smoothly connected at their connection portion, but a level difference is formed therebetween. At a bottom portion and its vicinity of the raceway groove of the slider, the surface of the raceway groove of the slider is located toward the center of curvature of the raceway groove of the slider with respect to the surface of the groove of the return guide, whereas at a shoulder portion and its vicinity of the raceway groove of the slider, the surface of the groove of the return guide is located toward the center of curvature of the raceway groove of the slider with respect to the surface of the raceway groove of the slider (see, for example, Japanese Patent Application Laid-Open No. 2015-197173).
There has been known a linear motion guide unit which has standard specifications for allowing use in wide applications, allowing easy formation of crowning, having a small crowning depth as compared with conventional linear motion guide units, and exhibiting high precision and durability. In the linear motion guide unit, at gateways to a raceway groove of a carriage, which raceway groove forms a load-carrying race, there are formed respective crownings having the form of gently curved surfaces for allowing rolling elements to smoothly enter and exit from the load-carrying race, and R chamfered portions in the form of curved surfaces formed at respective opposite ends of the raceway groove. The crownings and the corresponding R chamfered portions are connected continuously without formation of a corner portion (see, for example, Japanese Patent Application Laid-Open No. 2005-273765).
Meanwhile, the linear motion guide unit disclosed in the above Japanese Patent Application Laid-Open No. 2008-133837 relates to a reduction of sliding resistance and noise in high-speed motion and has a step portion such that a load-carrying rolling-element rolling surface is lower than a spacer inner circumferential surface used to form the turnaround passage of the slider. Therefore, in such a linear motion guide unit in which rolling elements are not retained by a retainer, the rolling elements collide against the step portion at a corner portion of an end surface of a spacer when the rolling elements roll into the turnaround passage from the load-carrying race. Also, the linear motion guide unit disclosed in the above Japanese Patent Application Laid-Open No. 2008-133837 intends to improve durability in high-speed running and is configured such that the rolling-element raceway groove of the slider has the first crownings, the second crownings, and the sloped surfaces at respective opposite end portions thereof and such that an intersection point of the slider end surface and an extension line of the second crowning coincides with an intersection point of the slider end surface and an extension line of a return guide; i.e., an extension line of the spacer inner circumferential surface. Therefore, in the linear motion guide unit, since a connection portion of the second crowning and the sloped surface protrudes from the return guide, exfoliation or flaking may occur as a result of rolling elements repeatedly colliding at high speed against the connection portion; further, since the two crownings must be formed by mechanical grinding, and the connection portion must be manually finished for R chamfering, a manufacturing process consumes time and labor, potentially resulting in an increase in cost.
The linear motion guide unit disclosed in the above Japanese Patent Application Laid-Open No. 2015-197173, which exhibits low noise and operates well, is configured such that regarding the positional relation between the surface of the raceway groove of the slider and the surface of the groove of the return guide, at a bottom portion and its vicinity of the raceway groove, the slider's raceway groove surface protrudes toward the center of the raceway groove, whereas at a shoulder portion other than the bottom portion and its vicinity of the raceway groove, the return guide's groove surface protrudes toward the center of the raceway groove. In the linear motion guide unit, since the bottom portion and its vicinity of the raceway groove of the slider protrude toward the center of the raceway groove as compared with the return guide, when rolling element move at high speed toward a rolling element rolling passage from the turnaround passage in high-speed motion, the rolling elements repeatedly collide against the protruding portion at an end portion of the raceway groove of the slider, potentially resulting in damage to the protruding portion.
The linear motion guide unit disclosed in the above Japanese Patent Application Laid-Open No. 2005-273765 exhibits high precision and durability and is configured such that the raceway groove of the carriage has crownings and R chamfered portions formed at respective opposite ends thereof and such that the crownings and the R chamfered portions are connected continuously without formation of a corner portion. In the linear motion guide unit, since the position of the spacer in relation to the raceway groove of the carriage changes within a dimensional tolerance, in the case where the carriage is higher than the spacer, rolling elements repeatedly collide at high speed against an end portion of the carriage, potentially resulting in damage to the end portion; in contrast, in the case where the spacer is higher than the carriage, the rolling elements may possibly be snagged by an end portion of the spacer. In a conventional linear motion guide unit, a slider slides at high speed on a guide rail in a reciprocating manner via a plurality of rolling elements, or rolling elements, rolling on the guide rail; the slider includes a carriage and end caps disposed on respective opposite end surfaces of the carriage; and when the rolling elements move in a relatively reciprocating manner, the rolling elements collide against a corner portion of the carriage, potentially resulting in damage to the corner portion of the carriage.